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Credit Sprint's Guide to Chemistry Unit 8: Acids and Bases

AP® weight 11–15%CLEP® weight 3–6%

Acids donate protons and bases accept them. pH, Ka and Kb describe strength, buffers resist pH change, and titration curves show how a solution changes as acid and base are mixed.

Weak acid titration curveVolume of NaOH addedpH half-equivalence: pH = pKa equivalence: pH > 7 pH 7 buffer region

What you need to know

  • pH = −log[H⁺]; pOH = −log[OH⁻]; pH + pOH = 14.00 at 25 °C. Each pH unit is a tenfold change in [H⁺].
  • Strong acids and bases ionize completely: [H⁺] or [OH⁻] equals the concentration (watch the formula, as in Ba(OH)₂).
  • Weak acids: Ka = [H⁺][A⁻]/[HA]; for a small x, [H⁺] ≈ √(Ka·C).
  • Conjugate pair: Ka × Kb = Kw = 1.0 × 10⁻¹⁴. The conjugate base of a strong acid is negligibly basic.
  • Acid strength: HI > HBr > HCl for binary acids (bond strength); more oxygens on the central atom means a stronger oxyacid.
  • Salt solutions: cation of a weak base is acidic (NH₄⁺), anion of a weak acid is basic (F⁻, CH₃COO⁻); ions of strong acids and bases are neutral.
  • Buffers (weak acid plus conjugate base): pH = pKa + log([A⁻]/[HA]). Best near pH = pKa, with high concentrations.
  • Titrations: the equivalence point of a strong-strong titration is pH 7; weak acid with strong base is above 7. At half-equivalence pH = pKa.
  • Choose an indicator whose color change brackets the equivalence-point pH.
  • Lewis acids accept electron pairs; Lewis bases donate them.

Equations and rules

pH = −log[H⁺]
Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴
pH = pKa + log([A⁻]/[HA])
Ka × Kb = Kw

Exam traps

Watch forpH of a weak acid isn't −log of its concentration.
Watch forEquivalence point and endpoint are not the same thing.
Watch forDiluting a weak acid raises its percent ionization even though pH rises.
Watch forNeutral pH is 7.00 only at 25 °C.

Key terms

  • pH: −log[H⁺]; below 7 is acidic at 25 °C.
  • Brønsted-Lowry acid: A proton donor.
  • Conjugate acid-base pair: Two species that differ by one proton.
  • Ka × Kb: Equals Kw = 1.0 × 10⁻¹⁴ for a conjugate pair.
  • Buffer: A solution of a weak acid and its conjugate base that resists changes in pH.
  • Henderson-Hasselbalch: pH = pKa + log([A⁻]/[HA]).
  • Half-equivalence point: Where [HA] = [A⁻], so pH = pKa.
  • Amphiprotic: Can act as an acid or a base, like HCO₃⁻ or H₂O.
  • Lewis acid: An electron-pair acceptor.
  • Percent ionization: [H⁺] at equilibrium ÷ initial acid concentration × 100.
  • Indicator: A weak acid whose color changes over a pH range near the equivalence point.

Practice questions

Five of the 65 Unit 8 questions in Credit Sprint. Every choice is explained in the app.

1. What is the pH of 0.0010 M HCl?
  1. 1.00
  2. 3.00
  3. 0.0010
  4. 11.00
Show answer

3.00. HCl is a strong acid: [H⁺] = 1.0 × 10⁻³ M.

2. The ion product constant of water, Kw, increases with temperature. What is the pH of pure water at 50 °C, where Kw = 5.5 × 10⁻¹⁴?
  1. About 7.4 and the water is basic
  2. About 6.6 and the water is still neutral
  3. 7.0 and the water is neutral
  4. About 6.6 and the water is acidic
Show answer

About 6.6 and the water is still neutral. [H⁺] = [OH⁻] = 2.3 × 10⁻⁷, pH ≈ 6.6, but [H⁺] = [OH⁻].

3. What is Kb of the acetate ion if Ka for acetic acid is 1.8 × 10⁻⁵?
  1. 5.6 × 10⁻⁵
  2. 5.6 × 10⁻¹⁰
  3. 1.8 × 10⁻⁹
  4. 1.8 × 10⁻⁵
Show answer

5.6 × 10⁻¹⁰. Kb = Kw ÷ Ka = 1.0 × 10⁻¹⁴ ÷ 1.8 × 10⁻⁵.

4. Which of the following is a conjugate acid-base pair?
  1. H₃O⁺ and OH⁻
  2. HF and F⁻
  3. NH₄⁺ and NO₃⁻
  4. HCl and OH⁻
Show answer

HF and F⁻. They differ by one proton.

5. Which pair would make the best buffer at pH 9.2?
  1. Acetic acid and sodium acetate (pKa 4.74)
  2. NH₃ and NH₄⁺ (pKa of NH₄⁺ = 9.25)
  3. HCl and NaCl
  4. HF and NaF (pKa 3.2)
Show answer

NH₃ and NH₄⁺ (pKa of NH₄⁺ = 9.25). The pKa is closest to the target pH.